1. Field of the Invention
The present invention relates to rotary phase converters for converting a single-phase, alternating current to three-phase, alternating current. In particular, the invention relates to improved rotary phase converters having means for switching the pair of coils in each of the windings of the converter from parallel connection to series arrangement during start-up of the converter, and then switching the pair of coils back to the parallel connection when the converter comes up to its normal operating speed.
2. State of the Art
The prior art has recognized that single-phase power can be converted to three-phase current for powering three-phase motors. Devices that develop three-phase, alternating current from a single-phase power source include rotary, dynamoelectric converters comprising an induction motor powered by single-phase current and producing three-phase current. Examples of such rotary converters are shown in U.S Pat. Nos. 4,777,421; 4,719,560; 4,158,225; 4,656,575; and 3,387,202. The converters must be started and brought up to speed from the single-phase input current. Because the three-phase motor of the converter starts slowly, if at all, on single-phase current and comes up to operating speed very slowly, the prior art has found several ways of modifying the converter motor to improve the starting characteristics.
As shown from the prior patents mentioned above, the modifications have taken three different approaches. In one approach, as exemplified in U.S. Pat. No. 4,656,575, starting and run capacitors are connected from one of the single-phase inputs of the converter to the non-powered output lead of the converter. Elaborate switching means must be provided for switching the starting capacitance into the circuit during start up and out of the circuit when the converter has attained operating speed. Another approach, as exemplified in U.S. Pat. No. 4,158,225 has been to provide a high resistance rotor for the converter. The third approach, as exemplified in U.S. Pat. No. 3,387,202, used a separate starting motor to start the converter motor and bring the converter motor up to speed prior to supplying a three-phase output.
3. Objectives
A principal objective of the invention is to provide a rotary phase converter that can be used in driving large, three-phase motors from single-phase power lines, with the converter having a novel starting system that uses exceptionally low starting current while accomplishing rapid, acceleration of the converter to its operating speed.
A particular objective of the present invention is to provide a rotary phase converter having a novel starting system that operates effectively with a rotor of low resistance resulting in improved performance and increased life expectancy of the converter.
Another objective of the present invention is to provide a rotary phase converter having a novel starting system that does not require specialized electrolytic or dry cell capacitors in the converter circuitry during start up or the related control devices and switch gear necessary to remove the specialized capacitors from the converter circuitry during normal operation.
A still further objective of the present invention is to provide a rotary phase converter that will start every time consistently, even in extreme cold temperatures.